This invention relates to control systems and methods for controlling an inductive load, such as a motor. More particularly, the invention relates to an improved power amplifier and control method for controlling the angular velocity and angular acceleration of a DC motor. The system and method disclosed maintain a low ripple current in the motor, simplify the motor control circuitry, conserve power used within the motor, and provide improved motor braking capability.
Switching Servo Amplifiers (SSA) are commonly used in the prior art to supply the drive current to an inductive load, such as a DC motor. Often the output stage used for the SSA is the "H" or "transistor/diode" bridge. An H-bridge comprises 4 power transistors interconnected to form a bridge, with the servo motor being positioned in the center of the bridge, as depicted in FIG. 1. Each transistor Q1-Q4 has a "free wheeling" diode D1-D4 connected, in a reverse current direction, from the emitter to collector. The base voltages of the transistors are controlled by the SSA to turn the transistors Q1-Q4 on and off in the appropriate manner so as to cause a drive current, I.sub.M, to flow through the motor M in the desired direction. A current sense resistor R1 is positioned so that the current I.sub.M flows therethrough. A differential amplifier A1 senses the voltage across R1 and converts this voltage to an output signal that indicates the polarity and magnitude of the current I.sub.M. The most common method of switching the power transistors of the H-bridge is through the Pulse Width Modulation (PWM) method. In this method, a fixed frequency is typically used and the duty cycle of the waveform is varied to supply the desired drive current I.sub.M to the motor.
Using the prior art scheme shown in FIG. 1, two diagonal power transistors in the H-bridge, such as Q1 and Q4, are simultaneously switched on, thereby causing current I.sub.M to flow through the motor in the direction shown by the arrow. After a period of time, determined by the duty cycle of the PWM waveform, transistors Q1 and Q4 are turned off, and transistors Q2 and Q3 of the H-bridge are turned on, switching the direction of the voltage applied to the motor. Transistors Q2 and Q3 remain on for a period of time, again determined by the duty cycle of the PWM waveform. After this period of time, the second pair of transistors Q2-Q3 is turned off, and the first pair Q1-Q4 is turned on. This "on" and "off" cycle of alternate pairs of transistors is continuously repeated as the servo system controls the acceleration of the motor.
Because a DC motor is an inductive load, the current flowing therethrough cannot be changed instantaneously when an alternate pair of transistors in the H-bridge is turned on. Thus, even though the voltage applied across the motor may approximate an instantaneous change (at the time the transistor pairs switch), the current flowing through the motor assumes a more triangular, or saw tooth waveform. Further, because the torque, and therefore the acceleration, of a DC motor is proportional to the average, or DC, current flowing through the motor (the direction of the acceleration being determined by the direction of the DC current), the servo system achieves the desired torque or acceleration by monitoring the motor current I.sub.M and controlling the duty cycle of the PWM wafeform until the desired average DC current is obtained. Unfortunately, this switching action causes a large ripple current to be ever present in the motor, even though the average current may be the desired amount. That is, while the motor is in motion, drive current is continually being supplied by alternately switching the desired transistor pairs. Even when the motor is not accelerating or decelerating, motor current is beng supplied with the duty cycle of the PWM waveform being 50%, thereby causing the average motor current supplied to the motor to be zero.
Unfortunately, the constant frequency PWM type of controller commonly used in the prior art suffers from several disadvantages: (1) the alternate "on" and "off" cycle of alternate transistor pairs causes a large ripple current to be ever present in the motor, unless the switching frequency is increased; in which case more power is lost in the switching transistors; (2) the large ripple currents in the motor may cause acoustical noise by coupling to the mechanical support members of the servo motor; (3) the AC component of the ripple current dissipates power in the motor, causing the motor to be unduly heated up; and (4) the drive current I.sub.M must be sensed at all times, which sensing consumes still additional power. Further, because the drive current will change directions in the sensing resistor whenever the motor current direction is changed, the differential amplifier must respond to this alternating current direction without introducing undesirable offsets. It is apparent, therefore, that a need exists in the art for an improved PWM amplifier and method for controlling the same wherein the above described disadvantages are not present.